flutter - 将项目添加到 Dart/Flutter 中嵌套 map 内的列表中

Question

当我打电话时addItemToCart({"key": "val"}) ，我无法将项目添加到 Dart/Flutter 中嵌套 map 内的列表中。 var list = cart['items']给了我一个空值，但我初始化了类(class)成员 cart已经。

它到底有什么问题？

import 'package:flutter/material.dart';
import 'dart:collection';

class PData extends ChangeNotifier {
  dynamic cart = {"items": []};

  int itemCount = 0;
  String uid;
  String phoneNumber;

  void addItemToCart(val){
    var list = cart['items'];
    print("list ");
    print (list);

    list.add(val);
    notifyListeners();
  }

}

给出以下错误:

I/flutter ( 5763): list 
I/flutter ( 5763): null

════════ Exception caught by gesture ═══════════════════════════════════════════════════════════════
The following NoSuchMethodError was thrown while handling a gesture:
The method 'add' was called on null.
Receiver: null

尝试调用:add(_LinkedHashMap len:6)

Answer 1

您需要添加 .toList() 才能启动。请参阅下面的示例代码片段:

var list = cart['items'];
var realList = list.toList();
print("list ");
print (list);

realList.add("soap");
print(realList);  // This prints [soap]
realList.add("Pen");
print(realList); // This prints [soap, pen]